A potential oxide for magnetic refrigeration application: CrO2particles

The magnetocaloric effect (MCE) of half-metallic CrO 2 particles has been studied with respect to particle size on the nanometre scale. Results from superconducting quantum interference device (SQUID) measurements indicate that acicular CrO 2 particles with a length of 400 nm yield a large magnetic entropy change Δ S M of 5.1 J kg −1 K −1 at an applied field of 15 kOe and an adiabatic temperature change Δ T ad of 2.0 K near the Curie temperature (∼390 K). These results are among the highest for magnetic oxides, and are comparable to that for pure Gd. However, smaller CrO 2 particles with a length of 260 nm only exhibit Δ S M = 2.25 J kg −1 K −1 and Δ T ad = 0.95 K. The difference in MCE between these two sizes of CrO 2 particles results primarily from disordered spins on the surface of the particles. In addition, measurements and calculations of the specific heat capacity for the CrO 2 particles are presented. These results indicate that the total specific heat capacity is dominated by the magnetic specific heat contribution. Therefore, we believe that these CrO 2 nanoparticles may hold future promise in the development of new magnetic refrigerants.